The invention relates to an in vacuo spark generator for the spectrographic analysis, in the far ultraviolet range, of solid samples.
It is known that the chemical constitution of a sample is determined by the spectrographic study of the plasma formed by which energy electric discharges carried out in vacuo between the sample acting as an electrode and another electrode.
Such discharges are produced by a generator, also called spark source, for exciting a spectral area extending from the far ultraviolet (2000 A to a few tens of A) to the vicinity of the weak X-rays.
In order to melt and vaporize by means of an under vacuum spark part of the constituent material of a sample, metallic or not, a considerable energy (from 10 to 100 joules) must be freed during a very short period (from 1 to 10 s). As a result, the analysis conditions quickly deteriorate following wear of the sample and modifications in the physical chemical state of the neighbouring surfaces caused by the extremely high temperature which has developed.
Most of the time, the deterioration is so quick that not only a precise quantitative information is not obtained but that also the results of analyses carried out in succession are not comparable between themselves.
The difficulty increases still more when one attempts to titrate or dose the elements by studying the lines appearing in the far ultraviolet, the energies in action bringing very quickly about a plasma concentration such that the continuous background revealed by the spectrographic analysis quickly masks said lines.
Various types of in vacuo spark generators are known, the differences between them being their principle of operation.
In a first type of generator, so called of the Millikan spark type, the spark is produced in vacuo between two metallic electrodes, one of which being the sample, for a disruptive field of very high value of 10.sup.5 to 10.sup.6 volts per cm. For voltages currently used, the distance between the electrodes does not exceed 1 mm. The discharge obtained is poorly stable and difficult to reproduce due to the wear of the electrodes which increases rapidly their spacing. Due to this fact, the generator used for identifying lines of a high ionization degree cannot be used for quantitative analysis.
In a second type of in vacuo spark generator, so-called sliding spark generator, the spark is produced between two electrodes, one of which being the sample, which are in contact with an insulating or semi-conductor support. The disruptive field is from 10 to 20 times weaker than that which is necessary for the generator of the first type and the spark can be formed in vacuo over a length of a few centimeters.
The generator of this second type is used in in vacuo spectrometry for works relating to fundamental research. Due to the insulating support, the generator is not convenient for carrying out quantitative analysis since the insulation supplies its own spectrum in the spectral area of the analysis. Moreover, the development of the spark depends on the physical-chemical nature of the surface of the insulating material, which is modified in permanence due to the thermal effect and to the deposition of material.
In a third type of in vacuo spark generator, so-called initiated spark generator, the main spark produced between two electrodes, one of which being the sample, is initiated by a spark of weaker energy ionizing the medium interposed between the electrodes.
In a first group of generators of this type, used for quantitative analysis in the far ultraviolet spectrum, the initiation spark is a sliding spark. Such apparatuses have the disadvantage mentioned hereabove and inherent to the sliding spark generators: the reproductibility of the discharges is very low. Moreover the maintenance interventions are numerous. In all cases, it is impossible to titrate with such generators the sample elements which are also present in the insulating material, for instance the oxygen of the sample when the insulating material is alumine.
In a second group of generators of this third type, the initiation spark is a Millikan spark, and the object is either to energize the X-radiations (S. K. HANDEL in ARKIV FOR FYSIK-Stockholm 1964), or to obtain continuous spectra in the ultraviolet. The cathode and initiation electrode, placed in the vicinity of each other, are supported by insulators and one observes in the plasma both the spectrum of the sample and that of the insulating material.